The present invention relates to an image forming apparatus utilizing electrophotographic technology such as a copying machine, a printer, and a facsimile machine and, more particularly, to a fixing device provided with a peeling member for peeling a recording medium fed out of a fixing nip portion.
Widely used as a fixing device for fixing a toner image transferred onto a recording medium such as a paper sheet in an image forming apparatus utilizing electrophotographic technology is a fixing device in which a sheet with a toner image transferred thereto is passed through a nip portion between a pair of rollers consisting of a fuser roller and a pressure roller to thereby fuse the toner image to the sheet by means of heating by the fuser roller and compression by the two rollers.
Generally, in the fixing device of this type, since the toner image fused to the sheet comes into contact with the fuser roller, a roller having a surface coated with a fluoro resin having high release property is used. However, even if such a fuser roller is used, the fused toner easily adheres to the surface of the fuser roller because it is soft and highly viscous, and the sheet may wind around the fuser roller.
Conventionally, in Japanese Patent Unexamined Publication H11-184300, a fixing device has been proposed which is provided with a sheet-like peeling member to be contact with a fuser roller on the downstream side. The peeling member comprises a resin sheet or a metal sheet as a substrate and a fluoro resin layer which is formed, by coating or applying, on the surface of the substrate and around an end of the substrate to be contact with the fuser roller.
In the aforementioned conventional fixing device, however, since the peeling member is in contact with the fuser roller, there is a problem that the fuser roller may be scratched. Then, a method is conceived that the peeling member is arranged not to be in contact with the fuser roller. However, this method has a problem that a gap between the end of the peeling member and the fuser roller must be held always constant over the entire length in the axial direction of the fuser roller in order to peel off a recording medium from the fuser roller with constant force over the entire width of the recording medium.
Moreover, in the conventional fixing device, it is common that the fuser roller and the pressure roller are arranged one above the other to be aligned in the vertical direction, and the feeding direction of the recording medium is substantially the horizontal direction. Therefore, a force acts on the recording medium, going out of the nip portion, in a direction moving apart from the fuser roller because of gravitation so that the prospect of peeling the recording medium by the peeling member is low.
However, when the fuser roller and the pressure roller are aligned in substantially the horizontal direction and the feeding direction of the recording medium is substantially the vertical direction, the recording medium easily winds around the fuser roller. In case of dual-side printing, there is also a problem that toner on the pressure roller side corresponding to locations where no toner is transferred on the fuser roller is fused so that the recording medium easily winds around the pressure roller. It may be conceived that another peeling member is provided on the pressure roller side. However, similarly to the peeling member on the fuser roller side, there is a problem that a gap between the end of the peeling member and the pressure roller must be held always constant.
In case that the fuser roller and the pressure roller are aligned in substantially the horizontal direction and a method of feeding a recording medium out of the fixing nip upwardly is employed, the recording medium easily winds around the fuser roller so that the recording medium always comes in contact with the peeling member, that is, the frequency in use of the peeling member is high. In this case, the setting of the contact angle of the peeling member relative to the recording medium is important. If the contact angle of the peeling member relative to the recording medium is not good, the peeling performance of recording media should be poor and linear defects may be created on image.
In the conventional fixing device, the peeling member is in contact with the fuser roller, so there is a problem that the fuser roller may be scratched. Since the conventional peeling member 51 has a fluoro resin layer 51f formed on the surface of a substrate 51a which has a round portion R on its end as shown in FIG. 10(A), an end portion of the resin layer 51f coming in contact with the recording medium is removed due to repeated use over the years as shown in FIG. 10(B). In this state, the recording medium is scratched by the round portion R having a larger contact area so that toner may adhere to the round portion and linear defects may be created on image.
Moreover, since the fuser roller and the pressure roller are normally biased against each other by springs at the both ends of the rollers, the middle portion of the pressure roller bulges because the elastic layer of the pressure roller is thick. As a result of this, the fuser roller is deflected into an arc shape in the axial direction. Therefore, it is very difficult to control the gap between the peeling member and the surface of the fuser roller. In case that the fuser roller and the pressure roller are aligned in substantially the horizontal direction, the recording medium easily winds around the fuser roller so that the recording medium always comes in contact with the peeling member, that is, the frequency in use of the peeling member is high, thus further increasing the importance of the control of the gap.